Known systems for lubricating an engine circuitously pump oil over and around moving engine parts with friction bearing surfaces such as valves and piston rods. With the passage of time, this oil acquires various contaminants, which are often particulates that have been shaved off of the engine parts themselves. As these abrasive particles circulate through the engine with the oil, they cause additional particulates to be shaved off of the engine parts. Therefore, a filter is typically placed somewhere in the path of the oil flow. This filter typically contains a filter element sleeve made from a porous material through which the oil can flow. After the oil has entered the filter, it must pass through this filter element before exiting the filter again. As the oil flows through the filter element, the particulates that have accumulated in the oil are retained.
One problem with these filters is that the filter element becomes clogged with the particulates that it is designed to retain. When a large amount of particulates are retained, the oil passing through the filter is impeded, thereby decreasing the volume of oil exiting the filter. As the volume of the flow diminishes, parts of the machine or engine that are normally lubricated receive inadequate lubrication. In some cases, the filter element becomes completely blocked, and thus, oil ceases to flow through the filter altogether. This not only can result in serious damage to the engine, but can even cause the engine to seize.
Additionally, when the filter element becomes clogged and the flow of oil is restricted, the differential pressure across the filter element increases. Because the material used to make the filter element is often relatively weak for purposes of permeability or cost, the increased pressure will often cause the filter element to tear. When this occurs, the filter element will sometimes break apart, and pieces will be swept away with the oil, thereby adding to, rather than reducing, the amount of particulates in the oil that cause wear to the engine parts over which the oil flows.
Another problem with these filters is that, when an engine in a cold environment is started, the viscosity of the oil is high, and thus, it resists flowing through the filter element. Just as when the oil is prevented from flowing through the filter element when the element is clogged, when oil flow through the filter element is restricted due to increased viscosity of the oil, inadequate lubrication, no lubrication, or torn filter elements may result.
Therefore, it is advantageous to have a mechanism that permits the oil to bypass the filter element when oil is not able to flow through the element. Accordingly, several bypass valve assemblies for use in filters have been suggested, such as that disclosed in U.S. Pat. No. 4,622,136 to Karcey, which is assigned to the assignee of the present application and which is incorporated herein by reference. Such assemblies, which form a barrier between the space surrounding the outside of the filter element and the space inside the filter element, are typically responsive to an excessive amount of pressure in the space surrounding the outside of the element.
When a filter element through which the oil normally flows becomes clogged, or the oil cannot flow through the element because the oil is too viscous, the pressure in the filter housing in the space surrounding the outside of the filter element increases. The bypass valve responds to a certain predetermined amount of pressure by opening, thereby permitting the oil to bypass the filter element by flowing through this opening, into the space inside the filter element, and ultimately back out of the filter housing.
A very serious problem with these filters, however, is that they are very limited with respect to the amount of filtering that they can achieve. This results from the fact that much of the contamination present in the oil is metallic particulates. These particulates are heavy, and thus, an excessive amount of this particulate matter is likely to cause traditional filter elements, which are typically paper, to tear. When this occurs, the oil will not only return to the engine unfiltered, but may be yet more contaminated, as it will likely include pieces of the filter element itself. Alternatively, in order to prevent such ripping of the filter element, the bypass valve can be set to open in response to a lower amount of pressure, thereby causing a greater volume of oil to return to the engine unfiltered. However, because the step of flowing through the filter element, which would normally retain particulates in the oil, has been skipped, the oil will still contain the particulate matter.
Accordingly, several filter assemblies have been proposed to combat the problems resulting from the presence of excessive metallic particulates in the oil. One type of assembly that has been suggested is the use of a bypass valve in conjunction with a stainless steel filter element, such as that disclosed in U.S. Pat. No. 6,267,875 to Leo. This arrangement can be advantageous because the filter element is stronger than tradition paper filter elements. Therefore, it can withstand a greater amount of pressure, and thus, the bypass valve can be set to open only in response to a greater amount of pressure than would be feasible for a paper element. The extra time and pressure resulting from this greater threshold permits more oil to be forced through the filter element before the bypass valve opens.
Another type of assembly that has been proposed is the use of a bypass valve in conjunction with a magnet or magnets. The value of using magnets in an oil filter in order to attract metallic particulates in the oil is well known. Accordingly, it has been suggested to use a magnet in a bypass filter, such as that disclosed in U.S. Pat. No. 4,689,144 to Holmes. This arrangement can be advantageous because the magnet serves as a sort of fallback filtering mechanism, such that the oil still receives some filtering if the filter element is bypassed.
One disadvantage of these assemblies, however, is that they are complex, include many parts, or have parts that render the assembly difficult to disassemble and reassemble, thereby resulting is assemblies that are costly and/or difficult to clean or replace. Arrangements facilitating disassembly are especially important in bypass filters, as contaminated oil often flows through the bypass assembly, thereby clogging or damaging it.
A further disadvantage of these assemblies is that they do not employ arrangements that are optimal for both increasing the volume of oil that must flow through the filter element and increasing the degree of filtering of that oil, while, at the same time, also provide a backup filtering mechanism for oil that bypasses the filter element. While it is critical that some sort of back-up filtering mechanism is in place in order to remove some of the damaging particulates from the oil when the filter element is bypassed, it is also important to provide both magnetic and non-magnetic filtering to as much of the oil as possible before the bypassing mechanism is employed. This is because many of the particles that contaminate the oil are magnetic in nature, usually falling into one of two categories: ferromagnetic particles, such as Fe, Co, Ni, and other metals and metal alloys, and ferrimagnetic particles, such as magnetic oxides Fe3O4, γ-Fe2O3, various ferrites, CrO2 and the like. Accordingly, such particles can be attracted by magnets, such as are defined by the formulas SrFe12O19 or BaFe12O19. However, some are non-magnetic, such as metal oxides. In fact, as metallic particles enter the circulating oil, they experience an oxidation process and thereby become less magnetic. Additionally, a non-magnetic filtering device, such as traditional filter element, can only remove particles having at least a certain minimum size, because, in order to retain smaller particles, one must use a filter medium that would also decrease the flow rate of the oil. Accordingly, it is advantageous to have a secondary filtering device to remove the smaller particulates that the filter element could not retain.
What is desired, therefore, is an apparatus that both maximizes the amount of oil that is forced through the filter element before a bypass valve opens and maximizes the amount of filtering experienced by the oil that flows through the filter element, and simultaneously provides a back-up filtering mechanism for filtering the oil that does eventually bypass the filter element. What is further desired is an apparatus that is inexpensive to manufacture and easy to clean.